1. Field of the Invention
This invention relates to a coaxial dielectric filter whose main part is constituted of at least two coaxial dielectric resonators and is used in, e.g., the regeneration of signals and extraction of clock signals in regenerative repeaters of optical communication. More particularly, this invention relates to an improvement of a coaxial dielectric filter, so improved that the locational relation in assembly component parts can be adjusted simply and also the freedom of signal-withdrawing direction can be made larger to enable miniaturization.
2. Description of the Related Art
FIG. 9 is an equivalent circuit diagram of a typical dielectric filter making use of coaxial dielectric resonators, comprising a tubular main body provided with an outer conductor (layer) and an inner conductor (layer) on its periphery and inner wall, respectively. FIG. 10 is a plan view showing the structure of a dielectric filter, in particular, a conventional coaxial dielectric filter making use of capacitors (Japanese Patent Application Laid-open No.51-130141 and No. 52-96846). In the typical coaxial dielectric filter, as shown in FIG. 9 equivalent circuit diagram, component parts such as capacitors C1, C2 and C3 (or inductors) are disposed in order to attain electrical coupling between an input-side connector and one coaxial dielectric resonator a, between both coaxial dielectric resonators a (stage-to-stage) and between the other coaxial dielectric resonator a and an output-side connector. In the conventional coaxial dielectric filter, the capacitors are formed on a substrate of alumina or the like by a technique such as thick-film printing in order to attain the desired electrical coupling, and are connected through capacitor connecting leads as shown in FIG. 10. In FIG. 10, letter symbol a denotes a coaxial dielectric resonator; b, an input-output capacitor; c, a stage-to-stage capacitor; d, a metal casing which constitutes part of a cutoff waveguide and is provided with a concave holding part d'; e, an input-side connector; f, an output-side connector; and g, a capacitor connecting lead.
Now, in dielectric filters, when it is attempted to produce a coaxial dielectric filter having a narrow passband width, it is necessary to use a capacitor having a very small capacitance. In the above manner of electrical coupling making use of a substrate of aluminum, it is structurally very difficult to materialize capacitors having a small capacitance, and hence there has been a disadvantage that coaxial dielectric filters having a narrow passband width can not be mass-produced.
Accordingly, the present applicant has already proposed a coaxial dielectric filter in which a novel manner of electrical coupling not making use of any component parts such as capacitors is employed so that its structure can be simplified (see Japanese Patent Application Laid-open No. 8-88504).
More specifically, the above coaxial dielectric filter is constituted chiefly of, as shown in FIGS. 11A to 11C, i) a straight cutoff waveguide constituted of a holding part d' provided in a metal casing d and a metal cover d" which closes the open side of the holding part d', ii) two coaxial dielectric resonators a disposed coaxially or substantially coaxially in the cutoff waveguide at a certain interval and each comprising a tubular main body al provided with an outer conductor and an inner conductor on its periphery a2 and inner wall a3, respectively, iii) a rod-like input-side antenna i whose base end is supported by an input-side connector e fastened to the input-side wall surface of the cutoff waveguide and whose leading end stands close, or inserted, to the inside of the input-side inner conductor of the coaxial dielectric resonator a disposed on the input side, and iv) a rod-like output-side antenna j whose base end is supported by an output-side connector f fastened to the output-side wall surface of the cutoff waveguide and whose leading end stands close, or inserted, to the inside of the output-side inner conductor of the coaxial dielectric resonator a disposed on the output side. Incidentally, since as shown in FIG. 11C the outer conductor and inner conductor provided on the periphery a2 and inner wall a3, respectively, of the tubular main body a1 have a common axis a', the resonators are called coaxial dielectric resonators.
In the above coaxial dielectric filter, the electrical coupling corresponding to the coupling at C1 and C3 shown in FIG. 9 are attained respectively by making the leading end of the rod-like input-side antenna i (whose base end is supported by the input-side connector) stand close, or inserted, to the inside of the input-side inner conductor of the input-side coaxial dielectric resonator a, and by making the leading end of the rod-like output-side antenna j (whose base end is supported by the output-side connector f) stand close, or inserted, to the inside of the output-side inner conductor of the output-side coaxial dielectric resonator a. Also, the amount of electrical coupling of these can be changed by changing the extent to which the rod-like input-side antenna i and output-side antenna j are made to stand close, or inserted, to the insides of the inner conductors of the input-side and output-side coaxial dielectric resonators a to change the distance or area where the inner conductors of the input-side and output-side coaxial dielectric resonators a face the antennas i and J, respectively. Hence, the amount of electrical coupling at C1 and C3 can be adjusted by changing the extent to which the rod-like input-side antenna i and output-side antenna j are made to stand close, or inserted, to the insides of the inner conductors of the input-side and output-side coaxial dielectric resonators a. Incidentally, when adjusted by making the leading ends of the antennas close to the insides of the inner conductors of the coaxial dielectric resonators, the amount of electrical coupling is smaller than when the leading ends of the antennas are inserted to the insides of the inner conductors. Its value, however, can be made larger by setting small the internal diameter of the tubular cavities of the coaxial dielectric resonators.
As for the electrical coupling corresponding to the coupling at C2 shown in FIG. 9, it can be provided by making larger the distance between open sides on the side where the input-side and output-side coaxial dielectric resonators a face each other, when they are disposed in the cutoff waveguide. The amount of this electrical coupling attenuates exponential-functionally as the distance between the input-side and output-side coaxial dielectric resonators a is made larger in the cutoff waveguide. Thus, the amount of electrical coupling C2 between the input-side and output-side coaxial dielectric resonators a can be adjusted by changing the distance between the input-side and output-side coaxial dielectric resonators a facing each other.
The coaxial dielectric filter shown in FIG. 11A has a structure wherein at least two coaxial dielectric resonators a and the rod-like input-side antenna i and output-side antenna j are disposed in the straight cutoff waveguide, and hence it has necessarily a long size. Thus, there has been room for improvement more or less when coaxial dielectric filters are made small-sized.
Accordingly, the present applicant has already proposed a coaxial dielectric filter in which a flexure (a bent portion) is provided in the cutoff waveguide so that its length can be made smaller (see Japanese Patent Application Laid-open No. 10-98305).
More specifically, this coaxial dielectric filter comprises, as shown in FIGS. 12A and 12B, a cutoff waveguide q having a flexure p and in which the input-side and output-side coaxial dielectric resonators a, the rod-like input-side antenna i and output-side antenna j and so forth as described above are disposed to constitute a filter that functions in substantially the same manner as the straight type coaxial dielectric filter shown in FIG. 11A.
According to these coaxial dielectric filters developed by the present applicant, the filters can be made to have a simple structure because of employment of novel manners of electrical coupling and the number of component parts can also be made smaller, and hence have an advantage that coaxial dielectric filters with a narrow passband can be produced simply and at a low cost.
Now, in such coaxial dielectric filters in which two coaxial dielectric resonators are incorporated, the desired characteristics can be achieved when the degree of electrical coupling is well-balanced at three points between the coaxial dielectric resonator a disposed on the input side and the input-side antenna i, between the two input-side and output-side coaxial dielectric resonators a and between the coaxial dielectric resonator a disposed on the output side and the output-side antenna J. In other words, the resultant coaxial dielectric filter changes in characteristic values if it is ill-balanced even at any one point.
When the filter is made well-balanced at the three points, a high precision is required for the adjustment between the two coaxial dielectric resonators, compared with the adjustment between the coaxial dielectric resonators and each antenna. This is because the amount of electrical coupling attenuates exponential-functionally as the distance between the coaxial dielectric resonators is made larger as stated previously, and hence the adjustment of distance between the coaxial dielectric resonators affects characteristics greatly when the degree of electrical coupling is made well-balanced, compared with the adjustment of distance between the coaxial dielectric resonators and each antenna.
Meanwhile, the coaxial dielectric resonators incorporated in these coaxial dielectric filters are each constituted chiefly of a tubular main body al having a cross-sectionally quadrangular shape in appearance and having a cross-sectionally circular shape at its cavity, formed of, e.g., a barium or titanium type oxide ceramic, and conductor layers formed of thick-film silver paste, provided on the periphery and inner wall of the tubular main body a1 (see FIG. 11C) (the conductor layer provided on the inner wall is called the inner conductor, and the conductor layer provided on the periphery is called the outer conductor), and also has a structure wherein the both ends in the lengthwise direction stand vertical and the both ends are not provided with any conductor layers so as to be made electrically open.
Individual coaxial dielectric resonators before adjustment, to be incorporated in the coaxial dielectric filter, have specific scatterings of dielectric constant or dielectric loss of dielectrics and conductor loss of the outer conductor which are caused by scatterings of conditions in the manufacture of dielectric materials (e.g., scatterings of process size, and scatterings of density which occur when molded or baked). Accordingly, before they are incorporated in the coaxial dielectric filter, the ends of coaxial dielectric resonators produced are cut individually to make pre-adjustment so that their resonant frequencies are uniformed, thus, as a matter of course, the individual coaxial dielectric resonators have non-uniform length depending on the amount of cutting.
Accordingly, when the coaxial dielectric resonators are disposed in the cutoff waveguide, the adjustment of distance between them can be made with difficulty because of such non-uniform length of the individual coaxial dielectric resonators, and also even any slight deviation in the setting of the distance between the coaxial dielectric resonators may make it impossible to obtain coaxial dielectric filters having the intended characteristic values. Such a problem has been unsettled.
Moreover, in the case of the coaxial dielectric resonator shown in FIG. 12A, having a flexure p in the cutoff waveguide q, there is no problem so much when the flexure is set at an obtuse angle. However, as the angle set at the flexure p is made smaller, it becomes difficult to dispose face-to-face the two coaxial dielectric resonators a at their ends on the side adjacent to each other (i.e., adjacent-side ends), and hence the electrical coupling between the resonators tend to become weak, making it difficult to attain the electrical coupling in an extreme case. Such a problem also has been unsettled.